1. Field of the Invention
The present invention relates to a Cathode Ray Tube (CRT) display device, and method. More particularly, the present invention relates to a CRT display, with a step-up circuit, and method for the same.
2. Description of the Related Art
In general, a Fly Back Transformer (FBT) of a CRT display outputs up to 26 kV. Generally, there are two types of high voltage regulation circuits available to stably supply high voltage. One circuit is a separable regulating circuit, separately regulating a high voltage and a deflection coil (Deflection Yoke, DY), and the other circuit is an integrated regulating circuit, regulating the high voltage and the deflection coil together.
FIG. 1 is an integrated regulating circuit of a conventional CRT display.
As illustrated in FIG. 1, the integrated regulating circuit includes a FBT 110, a horizontal deflection coil 120, a deflection signal controller 130, and a step-up circuit 140.
The FBT 110 has a primary conductive coil 111 and a secondary conductive coil 112. The secondary conductive coil 112 has a comparatively greater turn ratio than the primary conductive coil 111, and increases the voltage applied to the primary conductive coil 111. The voltage at the secondary conductive coil 112 is then supplied to a cathode of the CRT.
The horizontal deflection coil 120 is combined with an end of the primary conduction coil 111 of the FBT. By having a ramped current, the horizontal deflection coil 120 deflects an electron beam, generated by an electron gun, so that the electron beam is caused to scan across a display tube of the CRT, from corner to corner.
The deflection signal controller 130 includes a transistor Q3, pull-up resistors R1 and R2, and a damper-diode D2. The damper-diode D2, typically embodied by a silicon diode, is used to constrain a free oscillation generated after a flyback period of the ramped current waveform of the horizontal deflection coil 120. The transistor Q3 uses a BJT (Bipolar Junction Transistor) and switches on and off the voltage applied to the horizontal deflection coil 120, responding to a control signal applied to a base terminal thereof.
A primary purpose of the deflection signal controller 130 is to drive the horizontal deflection coil 120. The horizontal deflection coil 120 has to be correctly charged to a proper current level to enable a scanning of the electron beam to proceed from left to right on the screen. A control of the current strength, in the horizontal deflection coil 120, controls the deflection of the horizontal deflection coil 120 and enables the scanning to proceed horizontally. The horizontal deflection coil 120 generates a magnetic field and causes the electron beam to be deflected by applying a magnetic force to the electron beam. Horizontal-size (H-size) and horizontal-linearity (H-linearity), respectively, control the degree of deflection and the speed of the deflection. The step-up circuit 140 supplies power to the horizontal deflection coil 120, to enable a continuous deflection operation.
The step-up circuit 140 may include a BJT (Bipolar Junction Transistor) Q1, a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) Q2, a diode D1, pull-up resistors R3 and R4, a capacitor C1, and an inductor L1.
A signal generated according to a PWM (Pulse Width Modulation) signal, applied to the BJT Q1, is sequentially applied to a gate terminal of the MOSFET Q2, as a control signal switching the MOSFET Q2. The PWM signal applied to the BJT Q1 is a waveform designed to alternate between a zero duty level and a half duty level. When the MOSFET Q2 is in an off state, that is, when the PWM signal inputted to the BJT Q1 is in the zero duty level, the capacitor C1 is charged with a voltage applied to a drain terminal of the MOSFET Q2. On the other hand, when the PWM signal is in the half duty level, the voltage charging the capacitor C1 is increased by an electromotive force stored in the inductor L1. When the voltage (Vcc) applied to the drain terminal of the MOSFET Q2 is 50V, a voltage of the capacitor C1 is 50V if the PWM signal is in the zero duty level, but the voltage of the capacitor C1 increases to 160V or 180V if the PWM signal is in the half duty level.
A main role of the pull-up resistor R3 is to inform an IC (integrated circuit), controlling the deflection, that the MOSFET Q2 is in an on-state. When current is flowing through the pull-up resistor R3, during the on-state of MOSFET Q2, the voltage at Bsense is increased. Thus, the deflection controlling IC senses the voltage at Bsense and controls the same to be a high voltage by changing the on/off state of the MOSFET Q2.
The step-up circuit 140, focusing on the MOSFET Q2, is efficient for controlling the high voltage. The voltage at the capacitor C1 is flexibly changed based on a frequency varying from 31 kHz to 70 kHz, for example, as well as a maxmum/minmum load amount, of the electron beam, thereby compositively changing the high voltage.
The magnetic force applied to the inductor L1 is dependent on the length of the on-duty state of MOSFET Q2, e.g., the magnetic force will increase as the length of the on-duty state increases. Thus, the voltage in the capacitor C1 can be changed by the MOSFET Q2 being in the on-duty state. The voltage in the capacitor C1 is conducted to the primary coil 111 of the FBT 110 and supplied to the horizontal deflection coil 120.
Such a CRT device is widely used in TV picture tubes, computer monitors, and the like. A relay is also used to control a system of the above CRT devices, though the relay may be harmful to the system because of a chattering noise, a time delay, and so on.
Referring to FIG. 1, the conventional CRT display devices have several problems, such as the horizontal deflection signal controller 130 suddenly stopping because of an error from the relay, being an inefficient system environment, and potentially causing the PWM signal of the step-up circuit 140 to be continuously applied. Consequently, the electromotive force stored in the inductor C1 may continuously increase, and the current flowing through the MOSFET Q2 may rise to a dangerous level, such that the MOSFET Q2 may actually be destroyed.